its got me pulling my hair out.
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Mikuni BS34 carb woes
Moderators: KeithZ1R, chrisu, paul doran, Taffus
Mikuni BS34 carb woes
as above my bike was i think left on the side stand for 8 yrs so I've stripped the carbs now 3 times all seems ok and done the diaphragm test and are all ok, 3 and 4 cylinders run fine and 2 intermittent and 1 fires hardly at all, had the plug out of 1 and it sparks fine, they are all new plugs.
its got me pulling my hair out.
its got me pulling my hair out.
They will need to be stripped completely to components and ultra-sonically cleaned. (several times possibly for any that are obviously blocked)
Blow out with an air line any number of times and confirm with a can of WD40 with a straw on it.
Its likely the pilot circuits you will be interested in together with the jet pressed into the bottom of each float bowl.
Remove the needle jet / emulsion tube as well as the covered pilot jets which cant easily be seen.
Finally spray through with an aerosol and confirm each entry point has an exit and each exit has visible flow.
Dont forget the filters sitting behind the float valve needle seats!
When youre done put a tiny blob of araldite back on the places where you see it now ie. covering the ball bearings which are pressed into the external passageways.
May work out cheapest to buy or hire or borrow and cleaner and the fluid
AL
Blow out with an air line any number of times and confirm with a can of WD40 with a straw on it.
Its likely the pilot circuits you will be interested in together with the jet pressed into the bottom of each float bowl.
Remove the needle jet / emulsion tube as well as the covered pilot jets which cant easily be seen.
Finally spray through with an aerosol and confirm each entry point has an exit and each exit has visible flow.
Dont forget the filters sitting behind the float valve needle seats!
When youre done put a tiny blob of araldite back on the places where you see it now ie. covering the ball bearings which are pressed into the external passageways.
May work out cheapest to buy or hire or borrow and cleaner and the fluid
AL
1981 J1
Hi Farmer,if the bike was that long on the sidestand still with fuel in you can bet your bottom dollar that the left hand side carbs in particular have blocked fuel/air ways and clogged jets,i take it you have checked all diaphragms lift and drop together with blow test or similar,these type of carbs can be a nightmare got a few t shirts,ultrasonic clean may be an option..Welcome to the Forum by the way..Paul J
PAULJAC47,,,,,"She may not look like much, but she's got it where it counts, kid."
-Han Solo
You can't polish a turd, but you can roll it in glitter
Salad is what real food eats.
Anon
PUM 673
-Han Solo
You can't polish a turd, but you can roll it in glitter
Salad is what real food eats.
Anon
PUM 673
hi Paul and Al,
thanks for replying, I have stripped everything out, and sprayed carb cleaner in every hole and blown through with an air line, every jet and emulsifier has been cleaned and I can see through every hole, the only thing I have not done is split the carbs away individually, petrol is getting into 1 & 2 because I have to drain it off when I take the carbs off. I have ordered some new o rings from America as that's the only place I could find some, but they have not turned up yet.
thanks for replying, I have stripped everything out, and sprayed carb cleaner in every hole and blown through with an air line, every jet and emulsifier has been cleaned and I can see through every hole, the only thing I have not done is split the carbs away individually, petrol is getting into 1 & 2 because I have to drain it off when I take the carbs off. I have ordered some new o rings from America as that's the only place I could find some, but they have not turned up yet.
If you have whetted the plugs on 1 and 2 and they have part fired, likely those plugs are now dead!!!
With carbs off can you see all four butterflies opening at the same time and by the same amount with a similar start point?
A bank of four vacuum gauges will show if there is a problem with valves on mis-firing cylinders.
Needless to say; vacuum is critical and leaking valves will cause problems in this regard.
Are all four air / fuel mixture needles (screws) the same length?
Under a magnifying glass have they got the same taper and length of tip?
Ignoring the correct setting for a minute; richen them up to five or six turns out to see if it will fire. Should sound a bit like running on choke!
There is an exit drilling in the butterfly end of the venturi in the roof.
Spraying through the air / fuel mixture screw holes with them removed should give a good flow of whatever liquid used.
If any are different then that needs to be addressed first.
Which 'O' rings are you replacing?
Airbox or individual filters?
Are the carb to head rubbers perished / split and are they glued to the head?
Ultimately a simple colourtune will be one of the best things to get them set up correctly.
Keep at it.
AL
With carbs off can you see all four butterflies opening at the same time and by the same amount with a similar start point?
A bank of four vacuum gauges will show if there is a problem with valves on mis-firing cylinders.
Needless to say; vacuum is critical and leaking valves will cause problems in this regard.
Are all four air / fuel mixture needles (screws) the same length?
Under a magnifying glass have they got the same taper and length of tip?
Ignoring the correct setting for a minute; richen them up to five or six turns out to see if it will fire. Should sound a bit like running on choke!
There is an exit drilling in the butterfly end of the venturi in the roof.
Spraying through the air / fuel mixture screw holes with them removed should give a good flow of whatever liquid used.
If any are different then that needs to be addressed first.
Which 'O' rings are you replacing?
Airbox or individual filters?
Are the carb to head rubbers perished / split and are they glued to the head?
Ultimately a simple colourtune will be one of the best things to get them set up correctly.
Keep at it.
AL
1981 J1
The 4 air/fuel mixture screws are set at 2 1/2 turns out and look the same, so far I have only disassembled 1 carb at a time with those screws out I do spray carb cleaner through then air line it, I have bench sync the carbs and done the float level at 22mm, the o rings are I think the drain screw, the filter where the float needle sits, I can`t remember the other 2, I have seen a mikuni carb jet cleaner tool also from America rather like the gas torch type, is it worth getting one ?.
I have not checked carb inlet rubbers and its on original air box but the airbox to carb rubbers are not connected yet.
I have not checked carb inlet rubbers and its on original air box but the airbox to carb rubbers are not connected yet.
Wouldn't bother with the jet cleaners, more harm than good unless you are going to accurately measure the flow between same jets.
Mikuni genuine standard jets are +- 10% each so can make a big difference if done properly.
Airbox to carb rubbers need to be connected because without them there is no pressure differential across the carbs and therefore a reduced vacuum to operate the slides.
They are a bastard to start with no filters on!!!!
If the rubbers are hard dont mess about buy new ones. Old ones will make you cranky!!!
Bench sync works well for slide carbs but these are a little different in that they respond to the actual vacuum they see. Therefore with slight differences in inlet vacuum, sometimes you need them to have slightly different base settings.
In a perfect world it wouldn't be that way but what you are experiencing has a root cause out of the ordinary. Just need to find out what!
Are all the choke plungers fully home?
Push the choke knob in and then manually press the plungers in turn to seat them.
AL.
Mikuni genuine standard jets are +- 10% each so can make a big difference if done properly.
Airbox to carb rubbers need to be connected because without them there is no pressure differential across the carbs and therefore a reduced vacuum to operate the slides.
They are a bastard to start with no filters on!!!!
If the rubbers are hard dont mess about buy new ones. Old ones will make you cranky!!!
Bench sync works well for slide carbs but these are a little different in that they respond to the actual vacuum they see. Therefore with slight differences in inlet vacuum, sometimes you need them to have slightly different base settings.
In a perfect world it wouldn't be that way but what you are experiencing has a root cause out of the ordinary. Just need to find out what!
Are all the choke plungers fully home?
Push the choke knob in and then manually press the plungers in turn to seat them.
AL.
1981 J1
The carb air box rubbers are shits, gone hard, I have tried hot air gun and they stay supple for seconds and I have big hands !, I want to keep the original air box, where is the best place to buy these rubbers and any spares really as I need some braided brake hoses as well, maybe even master cylinders.
why do 3 & 4 run fine ?, should I put the jets and bits out of them into 1 &2 to see ?.
Graham
why do 3 & 4 run fine ?, should I put the jets and bits out of them into 1 &2 to see ?.
Graham
Originally had mine done by my local Ag place and they mis-understood what was required and just cleaned them in one piece. My mistake.
There are also preferred liquids for different applications.
You will need to remove all the jets and components and if unlucky take them down to four seperate units. Get the jets etc cleaned in the tank at the same time.
I left the throttle butterflies in place as i thought i would break them getting them out but i have seen them removed too.
Watch out for the tiny ball bearings and springs in the bodies which locate the choke bar!!
Dont snap off the stubbs that suspend the floats and the pilot jets are hidden beneath small plugs in the float bowl area!!!
Swapping jets between bodies will likely not help as the blockages are in the labyrinth of drilled passageways that connect them (jets etc) together.
Connector rubbers and parts available from various suppliers.
Find a favourite from;
Saisei
Debbens
Cradley Heath
Z1Enterprises
Zpower
Ebay
Zfever
Got my braided lines made up at PDQ in Slough, master cyl repair kits came from ZPower as did my connector rubbers, K&N filter for standard airbox was from Debbens, carb repair parts from NRP i think, ultra-sonic cleaner was borrowed from a club member as was the recommended fluid.
AL
There are also preferred liquids for different applications.
You will need to remove all the jets and components and if unlucky take them down to four seperate units. Get the jets etc cleaned in the tank at the same time.
I left the throttle butterflies in place as i thought i would break them getting them out but i have seen them removed too.
Watch out for the tiny ball bearings and springs in the bodies which locate the choke bar!!
Dont snap off the stubbs that suspend the floats and the pilot jets are hidden beneath small plugs in the float bowl area!!!
Swapping jets between bodies will likely not help as the blockages are in the labyrinth of drilled passageways that connect them (jets etc) together.
Connector rubbers and parts available from various suppliers.
Find a favourite from;
Saisei
Debbens
Cradley Heath
Z1Enterprises
Zpower
Ebay
Zfever
Got my braided lines made up at PDQ in Slough, master cyl repair kits came from ZPower as did my connector rubbers, K&N filter for standard airbox was from Debbens, carb repair parts from NRP i think, ultra-sonic cleaner was borrowed from a club member as was the recommended fluid.
AL
1981 J1
Farmer wrote:The 4 air/fuel mixture screws are set at 2 1/2 turns out and look the same, so far I have only disassembled 1 carb at a time with those screws out I do spray carb cleaner through then air line it, I have bench sync the carbs and done the float level at 22mm, the o rings are I think the drain screw, the filter where the float needle sits, I can`t remember the other 2, I have seen a mikuni carb jet cleaner tool also from America rather like the gas torch type, is it worth getting one ?.
I have not checked carb inlet rubbers and its on original air box but the airbox to carb rubbers are not connected yet.
Farmer, are you trying to run bike without airbox connected? if so cv type carbs like these run rubbish without the airbox/filter and a good seal on the airbox covers..Paul J
just read als post,covered already...
PAULJAC47,,,,,"She may not look like much, but she's got it where it counts, kid."
-Han Solo
You can't polish a turd, but you can roll it in glitter
Salad is what real food eats.
Anon
PUM 673
-Han Solo
You can't polish a turd, but you can roll it in glitter
Salad is what real food eats.
Anon
PUM 673
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